An Investigation of Warm Carbon Stars.

Warm carbon stars have been investigated with narrow-band photometric systems designed to measure band strengths of NH, CN, CH, and C(,2), an index of the C('12)/C('13) ratio, and the color temperature. Model atmospheres of R stars, computed by Dr. H. R. Johnson and Dr. B Gustafsson, are used to interpret the observed molecular indices in terms of elemental abundances. A differential analysis of the observed molecular indices for R, CH, CH-like, and barium stars as compared to G and K giants reveals that the barium stars have slightly higher C/O ratios than normal field giants and, though they are N-enriched with respect to the sun, they generally are not as N-enriched as the field giants. On the basis of the limited data available, the nitrogen abundances of the R stars appear slightly lower than those of normal giants; four out of five CH stars studied have nitrogen enrichments lower than those of the field giants. The C/O ratios of the R stars are generally higher than those of the CH stars. The carbon isotope ratio photometry shows the C('13) strength varies widely within each sub-group of stars studied. A comparison of the photometry (which spans a range of (TURN)6000 A) with model atmospheres shows the models represent well the energy distribution of the R stars in the near IR, but are underblanketed in the UV. This discrepancy appears to be similar to that found for models of G and K giants. By matching theoretical and observed molecular indices we find the R stars to have nitrogen abundances somewhat lower than those of the giants. The observed C(,2) indices cannot be reproduced theoretically by severely C-depleted models even if the C/O ratio is as high as six. The C(,2) indices can be satisfactorily matched by models having solar or greater than solar carbon abundances.